LC filter cutt off frequency

Hi all,

I'm a newbie in filters and i need to design a second order filter to attenuate some noise frequency ranges..


What i want to know is a basic thing.. Is the two below configurations of L & C the same?

will they have the same cutoff frequency at f=1/2pi*(rootLC)??

1.Cap on right side of Inductor.


2.Cap on Left side of the inductor.

Please see the attachments..



Thanks & regards
Steve.

themaccabee said:

Hi all,
I'm a newbie in filters and i need to design a second order filter to attenuate some noise frequency ranges..

What i want to know is a basic thing.. Is the two below configurations of L & C the same?

Click to expand...

To be honest, is that really your question?
Do you know what a voltage divider is? My recommendation: Don`t start with filters.
Start with simple resistive networks to understand the basic current-voltage relationships.

The characteristics of both designs will depend on the source and load impedance. Without that you cannot define the equations for the corner frequency.

Keith

Hi, im just attaching some of my files i had used for simulation...
What puzzles me is both showing a cutoff close to 5K (appx)

I suppose the transfer function for these are as follows

1.Cap on right of inductor from the source

Vo/Vin= 1/[1+(l/R)s+LCs^2]

2.Cap on left of the inductor

Vo/Vin=Ls/[1+(L/R)s]

My doubt is is it feasible to place the cap left or right as our choice since the bode plot is giving a cutoff approximately the same in each case?

I know the transfer fucntion will be different i n the two cases..
What exactly can be the difference?Am i right on the transfer function equations??

Thanks & Regards
Steve

Depending on the chosen parts values the 3-dB-cutoff frequency may be app. the same in both cases. However, the order of the filter is not the same:
Second order for the first and first order for the second design (the capacitor does not contribute to the transfer characteristics).

Hi LvW,

Could you take a look below to help me to get the idea clearly,

I was really interested in EMI filter design for Common mode noise.I read a coil craft Application note "Common mode Filter Design Guide"
Link:-
https://www.dianyuan.com/bbs/u/59/2225531194573765.pdf

Here in figure 4 (page 2)the second order filter is described,Common mode noise is being applied at a second order filter and the output measured across RL.

But the same application note at figure 7 (page 4) depicts the second order filter with capacitor at the input side (ei) of noise signal.

I was confused again when when i went through the EMI filter described at
https://scholar.lib.vt.edu/theses/available/etd-41298-162051/unrestricted/ethesis2.pdf

(Page 57 figure 2.31)

Here also the common mode filter capacitor Cy is placed close to the Noise source
ie the input side of the noise signal.

Hope im not troubling you people..but im really confused.

It would be great if anyone could help me a little bit.

Thanks And Regards.

Steve.

Hi themaccabee,

both of the figures you have shown to us don't tell us the truth - that means they are not compatible with the information of the referenced document.
(No voltage source with source resistance of zero and no sigle ended operation).

In the document, there is an ac line to be filtered. That's a different story - and in this case (line filter principles), of course, you can place a shunt capacitor in front of the inductor to design a second order system.

Added after 1 hours 12 minutes:

Hi themaccabee,

did you realize that in Fig. 4 of the referenced document the input is a the RIGHT side ?

did you realize that in Fig. 4 of the referenced document the input is a the RIGHT side ?

Click to expand...

I did.

both of the figures you have shown to us don't tell us the truth - that means they are not compatible with the information of the referenced document.
(No voltage source with source resistance of zero and no sigle ended operation)

Click to expand...

I was trying to analyse the filter described in the references independenlty,section by section.Common signal affects on both the supply rails equally and they can be thought of as noise sources affecting each supply rail independently..ie i think the assumption single ended operation for analysis purpose is correct.Thats y i put 1Vpp,50Hz, signal for simulation( i didnt give much thought for source resistance though).Also the transfer function i ve calculated is arrived in the same way.

May be im wrong in these assumptions..Please correct me if im wrong.

in this case (line filter principles), of course, you can place a shunt capacitor in front of the inductor to design a second order system.

Click to expand...

In this case are you placeing the input signal infront of the capacitor?.If so can u plz tell me the transfer function of the second order filter that can be obtained?

Hi themaccabee,

in your first posting you have shown with 11.jpg a circuit with a capacitor directly across the input. As KEITH has already answered, for calculation of the transfer function and to verify the role of this capacitor one has to know the (possible) source impedance.
Now, from the circuit as shown in the document referenced by you, I've got the impression that the case with a capacitor across the input is not interesting anymore.
However, if you want to see the influence of such a capacitor you have to consider the source impedance Zs ; if Zs=0 this capacitor has no influence.